[Python-checkins] Update logging documentation: change cross-reference and add webapp r… (GH-94540)
vsajip
webhook-mailer at python.org
Mon Jul 4 01:07:13 EDT 2022
https://github.com/python/cpython/commit/a391b74dbed085e4c8b922c79dcf7a9d9e72d3df
commit: a391b74dbed085e4c8b922c79dcf7a9d9e72d3df
branch: main
author: Vinay Sajip <vinay_sajip at yahoo.co.uk>
committer: vsajip <vinay_sajip at yahoo.co.uk>
date: 2022-07-04T06:06:43+01:00
summary:
Update logging documentation: change cross-reference and add webapp r… (GH-94540)
files:
M Doc/howto/logging-cookbook.rst
M Doc/library/logging.rst
diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst
index 79b4069f03507..7f4fe3a63a702 100644
--- a/Doc/howto/logging-cookbook.rst
+++ b/Doc/howto/logging-cookbook.rst
@@ -714,6 +714,228 @@ which, when run, produces something like:
2010-09-06 22:38:15,301 d.e.f DEBUG IP: 123.231.231.123 User: fred A message at DEBUG level with 2 parameters
2010-09-06 22:38:15,301 d.e.f INFO IP: 123.231.231.123 User: fred A message at INFO level with 2 parameters
+Use of ``contextvars``
+----------------------
+
+Since Python 3.7, the :mod:`contextvars` module has provided context-local storage
+which works for both :mod:`threading` and :mod:`asyncio` processing needs. This type
+of storage may thus be generally preferable to thread-locals. The following example
+shows how, in a multi-threaded environment, logs can populated with contextual
+information such as, for example, request attributes handled by web applications.
+
+For the purposes of illustration, say that you have different web applications, each
+independent of the other but running in the same Python process and using a library
+common to them. How can each of these applications have their own log, where all
+logging messages from the library (and other request processing code) are directed to
+the appropriate application's log file, while including in the log additional
+contextual information such as client IP, HTTP request method and client username?
+
+Let's assume that the library can be simulated by the following code:
+
+.. code-block:: python
+
+ # webapplib.py
+ import logging
+ import time
+
+ logger = logging.getLogger(__name__)
+
+ def useful():
+ # Just a representative event logged from the library
+ logger.debug('Hello from webapplib!')
+ # Just sleep for a bit so other threads get to run
+ time.sleep(0.01)
+
+We can simulate the multiple web applications by means of two simple classes,
+``Request`` and ``WebApp``. These simulate how real threaded web applications work -
+each request is handled by a thread:
+
+.. code-block:: python
+
+ # main.py
+ import argparse
+ from contextvars import ContextVar
+ import logging
+ import os
+ from random import choice
+ import threading
+ import webapplib
+
+ logger = logging.getLogger(__name__)
+ root = logging.getLogger()
+ root.setLevel(logging.DEBUG)
+
+ class Request:
+ """
+ A simple dummy request class which just holds dummy HTTP request method,
+ client IP address and client username
+ """
+ def __init__(self, method, ip, user):
+ self.method = method
+ self.ip = ip
+ self.user = user
+
+ # A dummy set of requests which will be used in the simulation - we'll just pick
+ # from this list randomly. Note that all GET requests are from 192.168.2.XXX
+ # addresses, whereas POST requests are from 192.16.3.XXX addresses. Three users
+ # are represented in the sample requests.
+
+ REQUESTS = [
+ Request('GET', '192.168.2.20', 'jim'),
+ Request('POST', '192.168.3.20', 'fred'),
+ Request('GET', '192.168.2.21', 'sheila'),
+ Request('POST', '192.168.3.21', 'jim'),
+ Request('GET', '192.168.2.22', 'fred'),
+ Request('POST', '192.168.3.22', 'sheila'),
+ ]
+
+ # Note that the format string includes references to request context information
+ # such as HTTP method, client IP and username
+
+ formatter = logging.Formatter('%(threadName)-11s %(appName)s %(name)-9s %(user)-6s %(ip)s %(method)-4s %(message)s')
+
+ # Create our context variables. These will be filled at the start of request
+ # processing, and used in the logging that happens during that processing
+
+ ctx_request = ContextVar('request')
+ ctx_appname = ContextVar('appname')
+
+ class InjectingFilter(logging.Filter):
+ """
+ A filter which injects context-specific information into logs and ensures
+ that only information for a specific webapp is included in its log
+ """
+ def __init__(self, app):
+ self.app = app
+
+ def filter(self, record):
+ request = ctx_request.get()
+ record.method = request.method
+ record.ip = request.ip
+ record.user = request.user
+ record.appName = appName = ctx_appname.get()
+ return appName == self.app.name
+
+ class WebApp:
+ """
+ A dummy web application class which has its own handler and filter for a
+ webapp-specific log.
+ """
+ def __init__(self, name):
+ self.name = name
+ handler = logging.FileHandler(name + '.log', 'w')
+ f = InjectingFilter(self)
+ handler.setFormatter(formatter)
+ handler.addFilter(f)
+ root.addHandler(handler)
+ self.num_requests = 0
+
+ def process_request(self, request):
+ """
+ This is the dummy method for processing a request. It's called on a
+ different thread for every request. We store the context information into
+ the context vars before doing anything else.
+ """
+ ctx_request.set(request)
+ ctx_appname.set(self.name)
+ self.num_requests += 1
+ logger.debug('Request processing started')
+ webapplib.useful()
+ logger.debug('Request processing finished')
+
+ def main():
+ fn = os.path.splitext(os.path.basename(__file__))[0]
+ adhf = argparse.ArgumentDefaultsHelpFormatter
+ ap = argparse.ArgumentParser(formatter_class=adhf, prog=fn,
+ description='Simulate a couple of web '
+ 'applications handling some '
+ 'requests, showing how request '
+ 'context can be used to '
+ 'populate logs')
+ aa = ap.add_argument
+ aa('--count', '-c', default=100, help='How many requests to simulate')
+ options = ap.parse_args()
+
+ # Create the dummy webapps and put them in a list which we can use to select
+ # from randomly
+ app1 = WebApp('app1')
+ app2 = WebApp('app2')
+ apps = [app1, app2]
+ threads = []
+ # Add a common handler which will capture all events
+ handler = logging.FileHandler('app.log', 'w')
+ handler.setFormatter(formatter)
+ root.addHandler(handler)
+
+ # Generate calls to process requests
+ for i in range(options.count):
+ try:
+ # Pick an app at random and a request for it to process
+ app = choice(apps)
+ request = choice(REQUESTS)
+ # Process the request in its own thread
+ t = threading.Thread(target=app.process_request, args=(request,))
+ threads.append(t)
+ t.start()
+ except KeyboardInterrupt:
+ break
+
+ # Wait for the threads to terminate
+ for t in threads:
+ t.join()
+
+ for app in apps:
+ print('%s processed %s requests' % (app.name, app.num_requests))
+
+ if __name__ == '__main__':
+ main()
+
+If you run the above, you should find that roughly half the requests go
+into :file:`app1.log` and the rest into :file:`app2.log`, and the all the requests are
+logged to :file:`app.log`. Each webapp-specific log will contain only log entries for
+only that webapp, and the request information will be displayed consistently in the
+log (i.e. the information in each dummy request will always appear together in a log
+line). This is illustrated by the following shell output:
+
+.. code-block:: shell
+
+ ~/logging-contextual-webapp$ python main.py
+ app1 processed 51 requests
+ app2 processed 49 requests
+ ~/logging-contextual-webapp$ wc -l *.log
+ 153 app1.log
+ 147 app2.log
+ 300 app.log
+ 600 total
+ ~/logging-contextual-webapp$ head -3 app1.log
+ Thread-3 (process_request) app1 __main__ jim 192.168.3.21 POST Request processing started
+ Thread-3 (process_request) app1 webapplib jim 192.168.3.21 POST Hello from webapplib!
+ Thread-5 (process_request) app1 __main__ jim 192.168.3.21 POST Request processing started
+ ~/logging-contextual-webapp$ head -3 app2.log
+ Thread-1 (process_request) app2 __main__ sheila 192.168.2.21 GET Request processing started
+ Thread-1 (process_request) app2 webapplib sheila 192.168.2.21 GET Hello from webapplib!
+ Thread-2 (process_request) app2 __main__ jim 192.168.2.20 GET Request processing started
+ ~/logging-contextual-webapp$ head app.log
+ Thread-1 (process_request) app2 __main__ sheila 192.168.2.21 GET Request processing started
+ Thread-1 (process_request) app2 webapplib sheila 192.168.2.21 GET Hello from webapplib!
+ Thread-2 (process_request) app2 __main__ jim 192.168.2.20 GET Request processing started
+ Thread-3 (process_request) app1 __main__ jim 192.168.3.21 POST Request processing started
+ Thread-2 (process_request) app2 webapplib jim 192.168.2.20 GET Hello from webapplib!
+ Thread-3 (process_request) app1 webapplib jim 192.168.3.21 POST Hello from webapplib!
+ Thread-4 (process_request) app2 __main__ fred 192.168.2.22 GET Request processing started
+ Thread-5 (process_request) app1 __main__ jim 192.168.3.21 POST Request processing started
+ Thread-4 (process_request) app2 webapplib fred 192.168.2.22 GET Hello from webapplib!
+ Thread-6 (process_request) app1 __main__ jim 192.168.3.21 POST Request processing started
+ ~/logging-contextual-webapp$ grep app1 app1.log | wc -l
+ 153
+ ~/logging-contextual-webapp$ grep app2 app2.log | wc -l
+ 147
+ ~/logging-contextual-webapp$ grep app1 app.log | wc -l
+ 153
+ ~/logging-contextual-webapp$ grep app2 app.log | wc -l
+ 147
+
+
Imparting contextual information in handlers
--------------------------------------------
diff --git a/Doc/library/logging.rst b/Doc/library/logging.rst
index 51b5f86e69efe..8716724af5625 100644
--- a/Doc/library/logging.rst
+++ b/Doc/library/logging.rst
@@ -233,7 +233,7 @@ is the module's name in the Python package namespace.
2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset
The keys in the dictionary passed in *extra* should not clash with the keys used
- by the logging system. (See the :class:`Formatter` documentation for more
+ by the logging system. (See the section on :ref:`logrecord-attributes` for more
information on which keys are used by the logging system.)
If you choose to use these attributes in logged messages, you need to exercise
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